Yeeun Kima,
Wonhee Leeab,
Gi Mihn Kima and
Jae W. Lee*a
aDepartment of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea. E-mail: jaewlee@kaist.ac.kr
bClimate Change Research Division, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea
First published on 31st May 2016
This paper describes the derivation of boron–manganese–carbon nanocomposites by CO2 carbonization using sodium borohydride (NaBH4) as a reduction agent at 1 bar, followed by impregnation of boron-doped porous carbon (BPC) with a form of manganese oxide (MO). The prepared composites (BPCMO) can be used as an advanced electrochemical energy material, such as active electrocatalysts for oxygen reduction reaction (ORR) and as electrode materials for supercapacitors. Various spectroscopic and microscopic measurements were carried out to investigate the morphology and structure of the BPCMO. Among many types of manganese oxide, it was confirmed that only Mn3O4 was embedded in the BPC. Cyclic and linear sweep voltammetry indicated that the BPCMO exhibits a four electron transfer pathway and has electrocatalytic activity comparable to that of commercial Pt/C. Galvanostatic charge/discharge and electrochemical impedance spectroscopic measurements showed that the BPCMO provided remarkable capacitance (150 F g−1 at 1.0 A g−1 and 136 F g−1 at 10.0 A g−1) compared to that of BPC (58 F g−1 at 1.0 A g−1 and 15 F g−1 at 10.0 A g−1), with a highly stable capacitance retention of 93.9% over 3500 charge/discharge cycles. It was found that impregnation of BPC with Mn3O4 enhanced electrochemical performance by generation of new active sites, increase in specific surface areas, and reduction of overall resistance.
In particular, carbon materials provide attractive features such as versatility, good electrical conductivity, chemical stability, large specific surface area, high porosity and low cost.9–12 Up to now, a myriad of approaches have been reported: nature-inspired materials such as bamboo,13 peanut shells,14 cotton,15 watermelon,16 human hair17 and microalgae18 have been used as carbon sources for the synthesis of porous carbon. While these attempts have shown reasonable improvements of electrode materials in supercapacitor and fuel cell applications, such source materials are not always easy to come by. Meanwhile, carbon dioxide (CO2) is a ubiquitous source that surpasses other candidates in terms of accessibility and abundance. Such abundance has led to enormous academic interest in not only reducing CO2 emissions, but also converting CO2 to carbon materials using boron-containing hydrides.19–22 As a result, the CO2 conversion field has received much public attention in contemporary society.23
In previous studies, it has been reported that boron-doped porous carbon materials can have a positive effect on electrocatalytic activity.20,24–30 In particular, the doping of heteroatoms such as phosphorous,31 nitrogen32 and sulfur33 into carbonaceous materials is one strategy to improve electrochemical performance. The effect of heteroatom-doping has been attributed to the differences in electronegativity between carbon and the heteroatoms. An atom that has relatively higher electronegativity than the adjacent atom draws electrons, resulting in asymmetric electron density. Consequently, this creates a positively charged site on the atom with lower electronegativity. This could induce new chemical and electrical properties such as favourable adsorption of oxygen, enhanced mass transfer capability, and pseudocapacitance, resulting in significantly improved electrochemical performance of supercapacitors and fuel cells.5,11,31–34
Another strategy to enhance the electrochemical performance of carbon materials for energy storage applications involves doping electrode materials with transition metal (TM) oxides. The availability of various oxidation states of transition metals facilitates the use of four-electron pathways to enhance oxygen reduction reactions (ORR).35,36 Moreover, the pseudocapacitance provided by TM-oxides enhances the electrochemical performance of supercapacitors.37,38 Ruthenium oxides (RuO2) show promise due to their high specific capacitance (720 F g−1), good chemical stability, and high electrical conductivity. However, their cost, toxicity and the rarity of ruthenium limit its use in commercial applications.39 Various cost-effective alternatives to RuO2 have been developed, including such as Co3O4,40 MnO2,41 NiO42 and Fe3O4.43 Among such alternatives, manganese oxide (Mn3O4) shows promise due to its low toxicity, outstanding electrochemical properties, low cost, and abundance. Unfortunately, it suffers from flaws such as poor conductivity and a low diffusion coefficient,44 which originate from the innate structural properties of Mn3O4. A promising approach to address the issues with Mn3O4 is to synthesize composite electrodes with Mn3O4 nanoparticles anchored to highly conductive porous matrix materials that provide more reaction sites between active materials and electrolyte ions. To the best of our knowledge, a hybrid material composed of Mn3O4 and boron-doped porous carbon has yet to be investigated as electrode materials for electrochemical systems.
In this work, we have successfully incorporated a certain amount of Mn3O4 into boron-doped porous carbon via a two-step process. Boron-doped porous carbons were first synthesized from sodium borohydride via synchronous carbonization of gaseous CO2. Next, the as-prepared carbon materials were impregnated with Mn(NO3)2·4H2O that was further decomposed to form Mn3O4 at an elevated temperature. The electrochemical behaviours arising from boron-doped porous carbon incorporated with Mn3O4 promise to enhance the performance of electrocatalysts for oxygen reduction reactions and supercapacitors for energy storage.
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Fig. 2 TEM images of (a) BPC (scale bar: 50 nm), (b) BPCMO40 with the inset showing BPCMO40 at higher magnification (scale bar: 100 nm). |
To identify the specific type of manganese oxide and obtain more detailed structural information, X-ray diffraction (XRD) measurements were performed for BPC and BPCMO40 (Fig. 3 and S1 in ESI†). Of all the possible forms of manganese oxide, the BPCMO40 contains only Mn3O4, as evidenced by the distinct peaks indexed to a tetragonal spinel (JCPDS no. 24-0734) in the profile.45 In view of the carbon structure, the existence of amorphous carbon is evidenced by a broad peak at 2-theta values of ca. 25 for BPC, but the peak becomes slightly sharpened for BPCMO40. Thus, the BPCMO40 has more graphitized carbon structure after the impregnation and subsequent heat-treatment step. The result of Raman spectroscopy, in which ID/IG increases in graphitization of the amorphous carbon case, also supports this (Fig. S2, refer to ESI†).25,46 The value of ID/IG is 0.92 (BPC), 0.94 (BPCMO30), 0.94 (BPCMO40) and 0.97 (BPCMO50). It seems that thermal energy mainly contributes to the more graphitized carbon structure by comparing with control group (named as BPC850) which is heat-treated at 850 °C without the manganese precursor. Based on our previous work, the ID/IG value of BPC850 is 1.10.25 Interestingly, the formation of Mn3O4 tends to involve restraining the graphitization of the carbon structure because BPCMO has a lower value than that of BPC850.
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Fig. 3 XRD patterns of BPC and BPCMO40. (Filled squares and open squares indicate peaks of Mn3O4 and amorphous carbon, respectively.) |
To investigate the porosity of BPCMOx, N2 adsorption/desorption isotherm measurements were performed. In Fig. 4, all the isotherms exhibit type IV isotherm, with a mixed H1 and H3 hysteresis loop in the range of the relative pressure (0.25–1.0). This comes from capillary condensation phenomena occurring in mesopores.47 The wide relative pressure range indicates that the BPCMO materials have mesopores of various sizes. In addition, the condensation effect observed at higher pressure reveals the existence of macropores.48 Fig. 5 shows that the pore size distribution (PSD) of BPC and BPCMOs is in the range 0.5–99 nm, which implies the presence of micro, meso, and macropores. This range would allow internal surfaces to be wet with the electrolyte and to provide active sites for electrochemical reactions,49 facilitating the transportation of ions and electrons.50 The decreasing order of nitrogen sorption amount is BPCMO30, BPCMO50, BPCMO40 and BPC, which is in accordance with the tendency of specific surface area (SSA) (Table 1). When BPC is impregnated with manganese oxide, the BET surface area (SBET) increases from 515 (BPC) to 615 (BPCMO30) m2 g−1 due to the thermal oxidation process of the manganese precursor.51 The PSD in Fig. 5 shows identical profiles because all of the materials are variants of BPC. However, this is not the case for pore sizes of 0.59 and 0.77 nm. In fact, the impregnated Mn3O4 has greater pore volumes at these values, leading to higher SSA values than the SSA of BPC, which increases the surface area for ion adsorption.50 To ensure that this was not due to thermal effects, BPC was annealed at 850 °C for 2 h, the same synthesis conditions as for BPCMOx. The SBET of BPC was greater than the SBET of BPC850 (Table 1), demonstrating that the increase of SSA in the BPCMOx originates from the impregnation of Mn3O4.
Initial mass ratio of Mn precursor | C (at%) | O (at%) | B (at%) | Mn (at%) | SBET (m2 g−1) | Vtotal (cm3 g−1) | |
---|---|---|---|---|---|---|---|
a XPS data from Byeon et al.25 | |||||||
BPC850a | — | 82.77 | 11.45 | 5.78 | — | 137 | 0.63 |
BPC | — | 64.03 | 16.09 | 19.88 | — | 515 | 0.85 |
BPCMO30 | 0.3 | 88.81 | 9.23 | 0.48 | 1.47 | 615 | 0.87 |
BPCMO40 | 0.4 | 74.36 | 17.1 | 1.84 | 6.7 | 563 | 0.80 |
BPCMO50 | 0.5 | 81.6 | 12.28 | 1.99 | 4.13 | 598 | 0.80 |
In order to understand the composition and chemical bonding states in BPC and BPCMOx, X-ray photoelectron spectroscopy (XPS) was performed. Table 1 shows the elemental distribution from the XPS survey data, which supports the presence of B, C, O and Mn atoms. According to the result of B 1s, as the ratio of Mn precursor increases, the atomic percentage of boron increases, indicating the existence of correlation between boron and manganese. Fig. 6a displays the spectrum for Mn 2p in BPCMO40, where distinct peaks at Mn 2p3/2 (641.7 eV) and Mn 2p1/2 (653.3 eV) are clearly observable. Here, a notable difference of 11.6 eV can be observed, proving the presence of Mn3O4. In addition, a slight shift (left) toward carbon is observed. This implies some interaction between Mn3O4 and carbon within BPCMO40, as evidenced in the manganese–carbon composites.34 The deconvolution of the B 1s spectrum of BPC and BPCMOs (Fig. 6b) indicates the presence of oxygen and boron containing functional groups such as B4C (187.5 eV), BC3 (188.9 eV), BC2O (191 eV) and BCO2 (192.3 eV).24,27 Impregnation of the BPC with Mn3O4 affects the bonding types of boron: as more Mn3O4 is added, B4C and BC3 tend to decrease while the portion of BCO2 or BC2O increases (Fig. 6c and Table 2), implying increased oxidation of boron.25
B4C (at%) | BC3 (at%) | BC2O (at%) | BCO2 (at%) | |
---|---|---|---|---|
BPC | 83.24 | 15.00 | 1.76 | — |
BPCMO30 | 45.60 | 18.62 | 19.79 | 15.98 |
BPCMO40 | 15.83 | 20.39 | 38.30 | 25.48 |
BPCMO50 | 8.67 | 9.18 | 78.41 | 3.74 |
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Fig. 7 CV of (a) BPC, BPCMOx and Pt/C (20 wt%); (b) BPC, BPCMO40 and Pt/C (20 wt%) in Ar (dot line) and O2 (solid line) saturated NaOH at a voltage scan rate of 50 mV s−1. |
Fig. 7b shows the CV plots of BPC, BPCMO40 and Pt/C (20 wt%) in Ar and O2-saturated 1 M NaOH solution. The current generated from ORR, the difference in current between Ar and O2 gas, is higher than that of Pt/C. The rise in current density implies an increment in catalyzed ORR, which is attributed to the increased number of active sites exposed to oxygen molecules.36 However, in the presence of Ar a reduction peak was not observed, but a secondary peak at approximately −0.4 V (vs. Ag/AgCl) appeared. This peak was also observed in the presence of O2, and we attribute this to a reaction between metal oxide and electrolyte.53 Mn3O4 causes a redox reaction, as do the other manganese oxides. The charge storage reaction of Mn3O4 is very similar to that of MnO2.54,55 Thus, Na+ in the electrolyte may lead to the following reduction reaction of Mn3O4:
MnO1.33 + δNa+ + δe− ↔ MnO1.33Naδ | (1) |
Fig. 8 is a voltammogram of an RDE at a speed of 2500 rpm for BPC, BPCMOs and Pt/C (20 wt%). For less negative potentials, BPCMOs show abrupt increases in current compared to Pt/C (20 wt%). The half-wave potential (E1/2) for ORR (the ORR initiation potential) is 0.857 V (vs. RHE) for Pt/C, 0.803 V (vs. RHE) for BPCMO30, 0.805 V (vs. RHE) for BPCMO40 and 0.826 (vs. RHE) for BPCMO50. The co-utilization of B and Mn3O4 results in a positive shift from 0.657 in BPC to 0.826 in BPCMO50. The current density at −0.3 V (0.7 V vs. RHE, a common half-cell cathodic potential of a fuel cell (I−0.3 V)) of BPCMO40 is approximately −4.633 mA cm−2, a higher value than that of Pt/C (20 wt%) (−4.16 mA cm−2). The increase in available active sites contributed to the increased current density.
The electron transfer number (n) and the amount of hydrogen peroxide (H2O2) produced by each product were gained from RRDE measurements in O2-saturated 1 M NaOH electrolyte (see the ESI† for details of the calculations). As shown in Fig. 9, BPCMO40 exhibits the highest ORR activity among the BPCMOs in the entire range of −0.3 V to −1.0 V. The range of n for BPCMO40 is 3.5–3.8, which is quite close to that of Pt/C in the range of potential between −0.3 V and −1.0 V, indicating that it follows a four-electron transfer pathway. Unlike BPCMOx, the electron transfer number is 1.8–2.4 for bare BPC in the range of potential between −0.3 V and −1.0 V, indicating a two-electron transfer pathway, an undesired mechanism for ORR due to its slow reaction and peroxide generation. It is clear that the addition of Mn3O4 to the carbon plays a key role in enhancing the electrocatalytic activities. As shown in Fig. 9b, there exists an inverse relationship between n and H2O2 yield.
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Fig. 9 Electron transfer number (n) and the amount of peroxide (HO2−) produced, calculated from RRDE data at 2500 rpm and at a scan rate of 50 mV s−1. |
The values for CV, RDE and RRDE are summarized in Table 3. When BPC was impregnated with Mn3O4, the Epeak, Ipeak, E1/2, I−0.3 V and n−0.3 V values all increased. The ratio of oxidized boron increases, carbon and manganese begin to interact, and SSA increases. When combined, these results indicate the improved ORR performance is explained by an increase in the number of active sites. In particular, the results obtained for BPCMO40 were comparable to Pt/C. XPS results (Table 2) for BPCMO40 show that the most oxidized form of boron, BCO2, is present in significant amounts, accounting for the high ORR activity.25 Thus, the synergy between boron-doping and manganese oxide impregnation leads to more active sites from oxidized boron, ultimately showing highly enhanced electrocatalytic performance.
Samples | Epeak (V) | Ipeak (mA cm−2) | E1/2 (V) | I−0.3 v (mA cm−2) | n−0.3 v |
---|---|---|---|---|---|
BPC | 0.66 | −0.39 | 0.657 | −0.261 | 1.79 |
BPCMO30 | 0.79 | −2.69 | 0.803 | −3.863 | 3.01 |
BPCMO40 | 0.79 | −3.69 | 0.805 | −4.633 | 3.48 |
BPCMO50 | 0.79 | −2.12 | 0.826 | −4.067 | 3.15 |
Pt/C (20 wt%) | 0.86 | −1.81 | 0.857 | −4.16 | 3.96 |
Fig. 10a is the result of stability tests for BPCMO40 and Pt/C (20 wt%). Using the chronoamperometry technique, the test was performed at −0.7 V in O2-saturated 1 M NaOH. The current density of BPCMO40 decreased by 9.5% for 24000 s, whereas that of Pt/C decreased by about 20%. We also tested this material in the presence of methanol to check its tolerance to methanol crossover effects (Fig. 10b). LSV for Pt/C (20 wt%) shows huge oxidation current density at about −0.3 V on the LSV curve. However, BPCMO40 does not show oxidation current density, suggesting its tolerance to the alcohol fuel.
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Fig. 11 CV curves of BPC and BPCMOx with the range of potential −1.0 to 0.0 V, at a scan rate of 10 mV s−1. |
Galvanostatic charge/discharge (GCD) measurements were carried out to acquire more information on the capacitive performance of the BPCMOs. Fig. 12a shows the GCD curves of BPC and BPCMOs at a current density of 1 A g−1 in 6 M KOH electrolyte solution in the potential range of −1.0 to 0.0 V (vs. SCE). The increase in the charging and discharging time indicates that the capacitance of the BPCMO composite is greater than that of BPC, which agrees with the CV results. As shown in Fig. 12b, SC values were 59, 107, 118 and 150 F g−1 at 1 A g−1 for BPC, BPCMO30, 40 and 50, respectively (refer to the ESI† for the calculation of SC). The BPCMO50 retained the highest capacitance due to its pseudocapacitance. The SC for BPCMO50 was 150, 147, 142, 135, 133 and 136 F g−1 at current densities of 1, 2, 3, 5, 7 and 10 A g−1, respectively. BPCMO50 maintained almost constant SC at various current densities, indicating its high stability. In addition, the BPCMO series showed outstanding normalized capacitance (specific capacitance divided by the surface area): 17.4 (BPCMO30), 21.0 (BPCMO40) and 25.1 μF cm−2 (BPCMO50), which is better than that of the graphene-based porous carbon (∼9 μF cm−2).56
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Fig. 12 (a) GCD curves at 1 A g−1 in 6 M KOH and (b) SC of BPC and BPCMOs at different current density. |
For better understanding of the superior capacitive performance of BPCMOs, electrochemical impedance spectroscopy (EIS) analyses were conducted. The Nyquist plot was presented in Fig. 13. According to the analysis, the Nyquist plot is composed of a semi-circular part and a vertical part. In the high-frequency region, the intercept at the beginning of the semicircle is related to the electrical resistance of the electrolyte (Rs). The Rs is 0.275, 0.336 and 0.345 Ω for BPCMO40, 50 and 30, respectively. In addition, the semicircle radius corresponds to the faradaic charge transfer resistance (Rct). As shown in Fig. 13, the Rct of BPCMO40, 50 and 30 was 0.257, 0.259 and 0.217 Ω, respectively. Both Rs and Rct values of BPCMOx were very low, compared to the results from prior work focused on composites of graphene–Mn3O4 (Rs = 2.02 Ω, Rct = 1.06 Ω), carbon nanotube–Mn3O4 (Rs = 1.25 Ω, Rct = 1.1 Ω) and CNF–Mn3O4 (Rs > 4 Ω, Rct > 2 Ω) for supercapacitor electrodes.45,57,58 In the low-frequency range, the linear part is related to the ion diffusion and transport in the electrolyte. For BPCMOx, the slope of the plot was steeper than for BPC, demonstrating that BPCMOx have faster diffusion of electrolyte ions within the pores of electrodes during redox reactions, which leads to better capacitive behaviour. Furthermore, the long-term cycling performance of the BPCMO50 was performed by the GCD at a current density of 2 A g−1 (Fig. 14). During 3500 cycles, only 6.1% capacitance loss was observed, indicating its viability for practical applications. The unchanged SEM image (Fig. S11 in ESI†) of the spent sample after the cycling test also supports the high stability of the composite.
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Fig. 13 Nyquist plots of BPC, BPCMOs using a sinusoidal signal of 10 mV over the frequency range of 100 kHz to 0.05 Hz. |
Footnote |
† Electronic supplementary information (ESI) available: Raman, XRD and electrochemical analysis data of BPCMOs. See DOI: 10.1039/c6ra10061a |
This journal is © The Royal Society of Chemistry 2016 |